As semiconductor microprocessing technology advances, wireless communication devices and apparatuses are stably supplied at a low cost and in a large quantity, becoming widespread in daily life. From now on, it is expected that a ubiquitous era will arrive when wireless communication devices are mounted on every entity. Since wireless communication devices in such ubiquitous era need to operate for long hours on a battery, reduction of the power consumption thereof has become an essential technology.
As one example of such wireless communication devices, a wireless device specification as shown in Non-patent Document 1 has been proposed. In FIG. 1 of Non-Patent Document 1, a configuration is shown in which a network is formed between wireless communication devices and in which data acquired by each device is sent to a server (a PAN coordinator). Also, in Patent Document 1, there is demonstrated a system in which a wireless communication device respectively mounted on a room temperature sensor, an air conditioner, and a hot-water supply apparatus transmits its state/operation to an external communication line, and the respective operational state is transferred by external means of communication to an external communication line. Further, in Patent Document 2, it is shown that a different frequency is used during transmission and during reception in a wireless communication device.
However, in the wireless communication systems disclosed in the above described Patent Document 1, Patent Document 2, and Non-patent Document 1, a problem exists in that a high-speed data communication and a power-saving operation are not consistent with each other. This is because the maximum transmitting/receiving rate of a wireless communication device is determined by the frequency band of the carrier wave to be used, while the power consumption thereof significantly depends on the frequency of the carrier wave.
That is, when performing a high-speed data communication such as image transfer, it is necessary to use a high frequency band such as a 5 GHz band or 2.4 GHz band; however, in order to extract data communicated in such a band from the carrier wave or to superimpose communication data on the carrier wave, a circuit which operates at a high-speed such as of 5 GHz or 2.4 GHz in the wireless communication device is necessary and such high-speed operation circuit part will consume the bulk of the power required for the wireless communication device.
Generally, in a wireless communication system, as shown in FIG. 1, a carrier wave of a single frequency band which is allocated by legal regulation such as a radio law is used to perform the transmission/reception of data and operational control. Therefore, the maximum transmission/reception data rate is determined by the allocated frequency band.
In the related art described in Patent Document 2, although separate carrier frequencies are used during transmission and during reception of a wireless communication device, the difference between the carrier frequencies is very small since it is based on the use of separate channels in the same frequency band, and therefore the difference in carrier frequency caused by selecting of different channels has little effect on the operational power.
Further, in Non-patent Document 1, a mechanism such as an intermittent operation is proposed to realize a low-power operation; however, in this system which utilizes a single frequency band, the total data communication amount will be reduced in proportion to the power decrease that results from the intermittent operation.
In recent years, an ad hoc network system which is realized only by terminals communicating with each other has gained much attention. This ad hoc network system is a system which is made up of, as shown in FIG. 2, a plurality of terminals 91 and communicates information with other terminals by exchanging transfer-data with each other on wireless transmission/reception signal 92 between terminals 91, the system being characterized in that it is not restricted by an access area and needs no expensive facilities.
A wireless communication network system which utilizes an ad hoc mesh network is widely used in a security ensuring system that utilize a monitor camera network, an entrance/exit inspection system, a physical distribution management system, a patient management system in medical fields, and the like.
Next, description will be made on a problem in the case in which a wireless transmitting/receiving apparatus that utilizes only one frequency band as shown in FIG. 1 is used as the terminal of the above described ad hoc network system.
First, a hidden terminal problem in an ad hoc wireless system will be described with reference to FIG. 3. In FIG. 3, terminal B and terminal C are in communication, and when terminal A is not aware that terminal B is in communication with terminal C, terminal A may send a communication request to terminal B which is performing communication. However, since terminal A cannot receive any response from terminal B, the communication request operation of terminal A becomes a wasted operation thereby consuming power in effectually. A flow chart to describe this operation is shown in FIG. 4.
In FIG. 4, when terminal A issues a communication request to terminal B (step 111), since terminal B and terminal C are in communication, terminal A cannot receive a response from terminal B (step 112). Therefore, terminal A is on standby for a fixed time period (step 113) and thereafter again issues a communication request to terminal B (step 114), thus repeating steps 115 and 116. These steps will be repeated until the communication between terminal B and terminal C is completed. Then, when communication between terminal B and terminal C is completed (steps 117 and 118), if terminal A will issues a communication request to terminal B (step 119), terminal A receives a response from terminal B (step 120), and communication will be started.
FIG. 5 shows an exposed terminal problem in an ad hoc wireless system. FIG. 5 shows a situation in which although terminal C and terminal D are in communication, and terminal B is trying to perform a data transmission to terminal A, since terminal B can perceive the communication of terminal C, it cannot go into transmission operation and cannot perform data transmission to terminal A. A flow chart to show this operation will be shown in FIG. 6.
When terminal B wants a communication with terminal A (step 131), terminal B inevitably detects the communication carrier of terminal C (step 132). Then, terminal B is on standby for a fixed period of time (step 133), and thereafter detects the communication carrier of terminal C (step 134) when again determining the presence or absence of the communication carrier, thus coming into standby operation (step 135). Then, this step is repeated until communication between terminal C and terminal D is completed. Then, when communication between terminal C and terminal D is completed (steps 136 and 137), terminal B confirms the absence of communication carrier (step 138), and is then allowed to issue a communication request to terminal A (step 139).
Thus, when an ad hoc network system is configured by using a conventional wireless transmitting/receiving apparatus, useless communication processing will be repeated and thereby transmission power will be wasted.
Further, a typical wireless communication system has a problem in which the time period of reception standby will be longest for each terminal, and when a high-frequency carrier frequency band is used in order to realize a high speed operation by increasing the transmitting/receiving transfer rate, it is necessary to operate a circuit for receiving a high carrier frequency even for standby operation and thereby power consumption will be increased.    Patent Document 1: Japanese Patent Laid-Open No. 2001-101578    Patent Document 2: Japanese Patent Laid-Open No. 10-13958    Non-patent Document 1: IEEE Computer Society, 804.15.4, Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs)
In the conventional transmitting/receiving apparatus described above, a problem exists in which high-speed data communication and a power-saving operation cannot be achieved at the same time.
It is an object of the present invention to provide a wireless transmitting/receiving apparatus and a wireless transmitting/receiving method which enables realizing both high-speed data communication and a power-saving operation.